To put it bluntly, Sonic the Hedgehog (2006) is a disgrace to the franchise.
To put the convoluted story in a nutshell: Sonic needs to protect this princess named Elise, because she is the key to unleashing hell (she releases it if she cries), from the clutches of Dr. "Eggman" Robotnik and a darker presence that looks like Shadow named Mephelis the Dark. Sonic also needs to keep at bay from a new hedgehog with psychokinesis named Silver, who believes that Sonic will bring the end of the world. There are a bunch of other characters as well, like Tails, Knuckles, and Amy, but they serve little to no purpose to the plot whatsoever. Most of the story pretty much makes no sense thanks to the "brilliant" use of time travel and the overall melodrama doesn't work for Sonic.
You have three hedgehogs to deal with: Sonic, Shadow, and newcomer Silver. Their goal for each level is simple: get to the goal ring as fast as you can. Sonic's gameplay is similar to that of the Sonic Adventure, whereas Shadow relies more on combat and vehicles much like his 2005 self-titled game (which are still pointless except for one or two areas) and Silver uses psychic powers to carry objects and enemies. A common element found in Sonic and Shadow's levels is that there is an overabundance of scripted events, which basically has the game playing itself with little to no input from whatsoever.
Occasionally (for whatever reason) you will switch over to one of the secondary characters, but they don't really serve any sort of purpose to the level and are simply there to make the levels feel more tedious and boring.
There are also a few incredibly bland and lifeless hub worlds full of weird citizens with pointless town missions. Unfortunately, a few of the town missions are required to progress through the story. Scattered throughout stages and the hub worlds are gold and silver medals, but aside from achievements they are pretty much worthless as well. By completing missions and stages you can collect rings that act as the game's currency. Later on in the game you'll be able to buy new abilities that were meant to have a limit to them, but they can be used as many times as you wish and can even be game breaking, such as being able to cross borders you weren't meant to get to. These help to deliver some mild amusement.
Aside from bad gameplay come the bad controls and camera. The controls are too sensitive, which makes it difficult to maneuver across platforms and turns. It's less of a hassle with Silver considering how slow he is. The camera is just as bad as the controls, especially in more enclosed areas where it likes to go haywire. Combine these with the bad gameplay and you get a thoroughly unpleasant experience for most of the game.
Now come the most infamous parts of the game: The level design and glitches. For example, when Sonic goes on a loop he magically stays glued on to the loop without falling off. So you can basically stand upside down or walk casually up the loop. If you don't judge your turns just right, scripted events can have you flying off to your doom. Your first encounter with Silver is very cheap. Once he grabs you, you will most likely die because of the fact that you are trapped within an invisible box where he can grab you again in a split second. When gliding, Knuckles or Rouge attach to a wall and be stuck for long periods of time until you reach an end or just get very lucky. Enemy and platform placement can come off as down right cheap, especially during super fast mach sections for Sonic. And of course you've got the classic pop-up and clipping problems, as well as long, tedious, and very frequent load times. There are plenty of other problems, but the list would drag on.
Graphically, it's rather disappointing by 360 standards. The majority of environments in the game are bland and character models have awkward animations. Some of the environments, however, don't look too bad, with Crisis City and Aquatic Base being the best looking levels. The FMV cutscenes change constantly, but a couple of them are pretty good and they are the most graphically pleasing part of the game. The music is really the best thing this game has going for it, but the voice acting is simply horrible. Delivery can either be forced or simply unemotional.
To sum it all up:
PROS:
-A few decent looking environments
-The music is really the best part of the game
-Some good CG
-Some amusing game breaking moments
CONS:
-Bad camera
-Over-sensitive control
-Pop-ups and glitches.
-Awful voice acting
-Secondary characters don't serve any sort of purpose other than to force you to play as them.
-Story is ridiculous
-Too many long loading times
-Pointless secondary missions
Overall, with just a few extra months (or perhaps another year or so) of TLC and the deletion of these unnecessary problems and additives, this would have been a big comeback for Sonic and a successful start to a bright future in Next-Gen consoles. But rushed production brought this game down hard and it is considered by many to be the worst game in the series.